Abstract
Under-frequency load shedding (UFLS) prevents a power grid from a blackout when a severe contingency occurs. UFLS schemes can be classified into two categories—event-based and response-driven. A response-driven scheme utilizes 81L relays with pre-determined settings while an event-based scheme develops a pre-specified look-up table. In this work, an event-based UFLS scheme is presented for use in an offshore standalone power grid with renewables to avoid cascading outages due to low frequency protection of wind power generators and photovoltaic arrays. Possible “N-1” and “N-2” forced outages for peak and off-peak load scenarios in summer and winter are investigated. For each forced outage event, the total shed load is minimized and the frequency nadir is maximized using particle swarm optimization (PSO). In order to reduce the computation time, initialization and parallel computing are implemented using MATLAB/Simulink because all forced outage events and all particles in PSO are mutually independent. A standalone 38-bus power grid with two wind turbines of 2 × 2 MW and photovoltaics of 7.563 MW was studied. For each event, the proposed method generally obtains a result with a smaller shed load and a smaller overshoot frequency than the utility and existing methods. These simulation results verify that the proposed method is practically applicable in a standalone power system with penetration of renewables.
Highlights
A power system must generate adequate power, such that the total power loads, losses, and required spinning reserves are met under normal conditions to maintain the system frequency at the rated value (60 or 50 Hz)
This paper proposes an event-based underfrequency load shedding (UFLS) scheme for a realistic standalone power system
A novel particle swarm optimization based on the least-upper bound method is proposed for the development of an event-based under-frequency load shedding scheme in this paper
Summary
A power system must generate adequate power, such that the total power loads, losses, and required spinning reserves are met under normal conditions to maintain the system frequency at the rated value (60 or 50 Hz). If an outage of one or more large power generators occurs in the power grid, a power mismatch between supply and demand is likely to occur, leading to a rapid decline in system frequency. A standalone or autonomous power system may experience a severe frequency drop if major power generation sources are lost on account of the small inertias of synchronous machines. Electric utility companies generally develop various protection schemes, such as underfrequency load shedding (UFLS), to deal with such severe events [1]. When appropriate loads are shed, the declining frequency can be restored gradually to its nominal value
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